Fire detection sensors are frequently designed as optical smoke sensors or smoke detectors. They function according to the Tyndall or diffused-light principle in most cases. Enumerated below are a number of documents on the state of the art that disclose various smoke detector assemblies: U.S. Pat. No. 4,242,673, U.S. Pat. No. 4,232,307, DE 27 54 139 A1, EP 0 076 338 A1, U.S. Pat. No. 4,180,742, and EP 0 360 126.
Error sources for the detection of smoke by means of such smoke detectors are parasitic light or diffused light that does not originate from smoke particles. Hence, light entering the detector casing from the outside can cause the generation of an alarm signal. The aspiration for such detector casings is that no external light at all or only very little light should get into the measuring length, if ever possible. However, since the detector casing has to present sufficient openings through which smoke particles might penetrate the entry of parasitic light cannot be avoided completely.
Another origin of parasitic light is a contamination of the detector chamber. Dirt which deposits on the walls of the detector casing leads to an intensification of diffused light. The higher the degree of contamination is, the more intense is the diffused-light fraction that is caused thereby. Therefore, the generation of an alarm signal must be expected from a certain degree of contamination onwards unless counteractions are taken. However, the generation of erroneously produced alarm signals should be avoided in any case because it could become expensive for the operator of a system of smoke detectors because a fire-brigade needs to be called. Therefore, EP 0 360 126 which was mentioned already proposes an efficient assembly by means of which a contamination of the measuring chamber walls is detected. This is accomplished by detecting and evaluating the reflection of an irradiated area of a measuring chamber wall. The degree of reflection increases with an increase in contamination. The measured value representative of a contamination can be used to adequately correct the threshold value so as to keep the sensitivity of the smoke detector approximately constant. By measuring the contamination of the measuring chamber, however, it is also possible to generate an alarm signal which preferably is provided to a detection control room in order that the contaminated detector be exchanged or cleaned.
A third error source consists in that a dew film forms within the smoke detector. In this case, small water droplets develop on dew film nuclei on the surface of the measuring chamber walls and on the optical elements, e.g. lenses or plastic bodies of the light transmitters or light receivers. In contrast, the electronic circuit for evaluating the measurement signals in the smoke detector can be protected against moisture very well and can be provided with a protective varnish or be embedded in a sealing compound.
The enhanced reflective property of the measuring chamber walls, when due to the moisture coat, generates an increased reception signal on the light-sensitive receiver. If no additional measures are taken the alarm threshold for smoke will be reached within a very short time and, hence, a false alarm will occur.
It is known to provide constructional steps to form a smoke chamber casing or a measuring chamber as well as a specific arrangement of the optical elements by which a dew film formation is supposed to be prevented. However, it is recognizable that a dew film formation ultimately cannot be prevented. This is why proposals were made to provide such detectors with a heater element. However, the heater element requires a corresponding amount of energy. Furthermore, it changes the behaviour of smoke penetration into the measuring chamber of the smoke detector.
Another possibility is to provide a humidity sensor which measures humidity in the area which directly surrounds the smoke detector. There is a hazard of the humidity sensor getting contaminated in case of an service life. As a result, there will also be errors in the measurement of air humidity. Furthermore, durable and long-life humidity sensors are relatively expensive. Finally, it is necessary to tune fire detectors including humidity sensors to each other while they are produced, which increases their manufacturing expenses.
DE 4 307 585 C1 has made known a method and apparatus for compensating humidity in a diffused-light detector. A moisture coating on the reception optics is detected by means of a further light transmitter and the light receiver that exists already by getting the smoke density periodically measured by the first light transmitter and humidity by the second light transmitter in a time-shifted manner. The two values measured are processed with the moisture coating reflecting the light of the second light transmitter while attenuating the receiver's output signal in dependence on the thickness of the moisture coating. An assembly of this type is also relatively expensive and does not absolutely safely lead to the target of avoiding the detrimental effect of a dew film.